A variety of different applications exist wherein it is desirable to combine information signals. For example, one typical application is the provision of subliminal messages. In order to present a subliminal message upon a television screen, it is necessary to provide apparatus for producing the data in the form of signals which are capable of being processed by a standard television receiver and to synchronize the signals with the R.F. signals being broadcast to the television receiver such as, for example, a transmission from a local or network television station to a television in a viewer's residence.
The transmitted video signal is typically comprised of synchronizing signals, namely, frame and line synchronizing signals also referred to as vertical and horizontal sync signals, respectively. The sync signals are combined with the image information to form a composite video signal which is transmitted to a television receiver by way of a carrier frequency signal which is modulated by the composite video signal.
The modulated carrier is processed at the television receiver to remove the composite video signal from the carrier frequency signal whereupon the video display is generated in accordance with the information signals and the corresponding synchronizing signals.
Heretofore, conventional techniques for displaying information such as messages, graphic and pictorial images and the like, utilize synchronizing signals which are generated specifically for the data to be superimposed, which signals are generated totally independently of the synchronizing signals forming part of the composite video signal.
One such conventional prior art technique is described in U.S. Pat. No. 4,616,261 in which switching means is provided for alternately switching between the composite video signal and the information to be combined therewith. This system has the disadvantages of cutting out the composite video signal during the time that the message, such as a subliminal message, is being introduced, and further requires separate, independent sync signals for the subliminal message, which switching system is thus incapable of precisely synchronizing the subliminal message with the video image as regards its rate of occurrence and precise location upon the screen.